Technical Field
The present invention relates to a photoacoustic image generation device and a photoacoustic image generation method, and more particularly to a photoacoustic image generation device and a photoacoustic image generation method that involves applying light to a subject, and generating a photoacoustic image by detecting acoustic waves that are emitted from the interior of the subject due to the applied light.
Background Art
Ultrasonography is known as one of imaging examination methods that allow non-invasive examination of state of the interior of a living body. In ultrasonography, an ultrasound probe that can transmit and receive ultrasound is used. Ultrasound transmitted from the ultrasound probe to the subject (living body) travels through the interior of the living body and is reflected at a tissue interface. Then, the reflected ultrasound is received by the ultrasound probe. Based on the time taken for the reflected ultrasound to return to the ultrasound probe, the distance is calculated, thereby imaging the state of the interior.
Further, photoacoustic imaging, which images the interior of a living body using the photoacoustic effect, is known. In photoacoustic imaging, in general, pulsed laser light is applied to the interior of a living body. In the interior of the living body, a living tissue absorbs energy of the pulsed laser light, and ultrasound (photoacoustic signals) is emitted due to adiabatic expansion caused by the energy. The photoacoustic signals are detected using an ultrasound probe, or the like, and a photoacoustic image is constructed based on the detected signals, thereby visualizing the interior of the living body based on the photoacoustic signals.
With respect to the photoacoustic imaging, Japanese Unexamined Patent Publication No. 2011-217767 (hereinafter, Patent Document 1) teaches that spatial frequency processing is performed. With the technique of Patent Document 1, light from a light source is applied to a subject, and photoacoustic signals are detected with a plurality of detector elements. The detected photoacoustic signals include photoacoustic signals emitted from the surface of the subject and photoacoustic signals emitted from the interior of the subject. With the technique of Patent Document 1, signals detected by the detector elements at the same reception time are subjected to a Fourier transform in the spatial direction to obtain spatial frequency signals. Then, components of frequencies not greater than a predetermined frequency of the spatial frequency signals are reduced. Then, the spatial frequency signals with reduced low-frequency components are subjected to an inverse Fourier transform, and a photoacoustic image is generated based on the signals obtained by the inverse transform. The photoacoustic signals emitted from the surface of the subject are simultaneously obtained with the plurality of detector elements, and therefore imaging with reduced influence of the photoacoustic waves emitted from the surface of the subject can be achieved.
With respect to the spatial frequency processing, Japanese Unexamined Patent Publication No. 7(1995)-282247 (hereinafter, Patent Document 2) teaches that image data obtained by ultrasound imaging is subjected to a Fourier transform to generate spatial frequency domain image data, and predetermined low-frequency components are removed from the Fourier-transformed spatial frequency domain image data. Then, the spatial frequency domain image data from which the low-frequency components have been removed is subjected to an inverse transform to generate image data of real data domain. Patent Document 2 teaches that the low-frequency components are equivalent to shading, and image portions that are necessary for diagnosis are not present in the low-frequency range, and therefore removing the components of low spatial frequencies is equivalent to removing shading. That is, in Patent Document 2, a desired area to be observed in the image data is present in the high-frequency range in the spatial frequency domain. Therefore, the low-frequency range in the spatial frequency domain is removed to remove areas other than the necessary area.